Preheat oven to 350° F.
Grease a 9 X 13-inch cake pan with vegetable oil spray.
In a small bowl, combine oats and boiling water; set aside.
In another bowl, mix flour, baking powder, baking soda, cinnamon and salt.
Set aside.
In a large bowl, cream together margarine and sugars until light and fluffy.
Beat in egg whites, orange juice concentrate and vanilla.
Blend in flour mixture alternately with oats, beginning and ending with flour.
Pour into greased pan and bake 25 to 30 minutes, or until cake springs back when lightly pressed.
Remove cake from oven and preheat broiler.
To make the topping, combine margarine, brown sugar and orange juice concentrate in a small saucepan.
Bring to a boil and cook 1 minute, over medium-high heat, stirring constantly.
Remove from heat and stir in nuts.
Spread topping evenly over warm cake; place under broiler for 1 minute, or until golden brown.
Topping can burn easily.
Watch it carefully! Allow to cool on rack before cutting.